U.S. Pat. No. 5,756,901 describes an acceleration sensor produced by surface micromechanics, which is provided with an encapsulation for protection from external influences such as humidity or dirt particles, as well as for the maintenance of a vacuum, and a layer system for producing such a sensor. In addition, German Published Patent Application No. 195 37 814 (DE 195 37 814 A1) describes a sensor and a method for its production, in which a movable sensor element is patterned out of a silicon layer with the aid of surface-micromechanical methods.
DE 195 37 814 A1 also describes a method for encapsulating sensor elements by the bonding of silicon caps by a seal-glass solder connection, which, however, on account of the flow behavior of the solder, requires a large surface reserve. Alternative, space-saving methods not having surface reserves are based on a so-called anodic bond process, in which Pyrex glass or a similar glass is connected to a silicon layer by applying an electric voltage of 1000 V or more, at a temperature of 400° C., for example, so that, by a charge separation in the glass (Na+ ions and O2− ions) a high-strength connection between the Pyrex and the silicon is created after an adhesion phase that is at first reversible.
It is customary, in the case of such an anodic bond of a cap or plate made of Pyrex glass over surface micromechanical sensor structures, to first prestructure it in such a way that it has a recess in the vicinity of the sensor structures, and subsequently to connect it hermetically sealed to the silicon from which the sensor structures were patterned out. However, in this context the problem arises that, because of the high electric voltage applied, the self-supporting and movable sensor structures are deflected electrostatically, and partially bond to the glass plate. This danger is increased further in that Pyrex glass demonstrates good adhesion properties to silicon, and stores electric charges on its surface which exert forces on the sensor structures even without external influence, and thus impair their function. Feed-through openings in the cap may be provided for a front side contact of the sensor structures that is frequently desired.
If, on the other hand, the recess is deepened to such an extent that the electrostatic attraction is reduced to a reasonable degree, such a cap is no longer able to act at the same time as the upper stop for the protection of the micromechanical structures generated, i.e. in the overload case, such as in case of mechanical shocks to acceleration sensor structures, the latter are deflected upwards, without hindrance, so far that they are destroyed. Moreover, structuring Pyrex glass is problematical when etching depths of some 10 μm are required.
In the encapsulation by anodic bonding of Pyrex glass to silicon, it is also disadvantageous that, in the process, oxygen is liberated from open glass surfaces, so that, in practice, the lowest pressures that may be enclosed under such caps is about 100 mbar, which is insufficient by far for rotational rate sensors made by surface micromechanics, which generally require working pressures of about 1 mbar. To overcome this problem, using materials which bind the oxygen in the cavity formed by the cap has been suggested. However, this procedure is expensive, and a great effort from a process technology point of view.
Additionally, in the case of encapsulation of microstructure components, one may use a silicon wafer as the encapsulation wafer including a glass layer on the surface, for example, a Pyrex glass layer. This is then ground to the desired thickness, polished, and then provided with structuring in the form of a cavity as the cap for the sensor element. Thereby, the outflow of oxygen into the interior of the cavity is reduced, and on the other hand, the electrically conductive silicon wafer which forms the actual cap may be electrically contacted even though this may be costly from a process engineering point of view, and this guards against the danger of electrostatic discharge. However, even in this specific embodiment, the danger of electrostatic collapse and the bonding of the capped, microstructured sensor elements to the Pyrex glass layer or an exposed silicon area on the bottom of the cavity in the Pyrex glass layer is still present. In addition, in this case too, many times the cap cannot act as a stop for limiting a vertical deflection of the encapsulated microstructures in the case of overload, since, as a result of the relatively high tolerances in the grinding and polishing, the thickness of the Pyrex glass layer, which determines the distance of the cavity bottom from the microstructure, is too great to effectively limit the deflection, or, on the other hand, is too small to be able to exclude an electrostatic collapse during the anodic bonding. Moreover, the grinding down of the Pyrex layer with great precision represents a considerable cost factor, usually requiring residual tolerances of +/−5 μm. This being the case, the residual thickness of this layer is designed to be at least 20 μm, which makes it ineffective as a stop. In addition, the structuring of Pyrex glass layers that are etched to be 20 μm thick by plasmas or hydrofluoric acid solutions is costly and time-intensive.